How to Fix Your Android Screen Not Rotating

Solution 1: Turn on Auto rotate. You’ll find this setting in the Quick Settings menu. If you see Auto rotate highlighted in blue, then auto rotate is turned on. If you don’t see Auto rotate, but there’s a Portrait icon instead, auto rotate is disabled. Tap Portrait to turn on auto rotate.

Solution 2: Restart your Android phone. Restarting your phone purges system memory, which often resolves many strange behaviors. It will not only resolve minor issues, like your Android screen not rotating, but could also fix a slow phone or fix a frozen screen.

Solution 3: Send the phone in for repair. The root cause could be faulty hardware and it might need professional repair.

Credit : https://www.lifewire.com/fix-android-screen-that-wont-rotate-5088086